Patella Tracking

ABSTRACT

Disclosed herein are a surgical system for patella tracking and a method for selecting a properly-sized patellar implant utilizing the same. The surgical system may include first and second trackers and a patellar tracking system. The first tracker may be configured to contact an unresected or a resected patella, and the second tracker may be configured to contact a bone. The patellar tracking system may be configured to track the first and second trackers during patellar flexion and extension to generate patellar range of motion and patellar trial range of motion. A method for selecting a patellar implant may utilize the first and second trackers and the patellar tracking system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 62/687,875 filed Jun. 21, 2018, thedisclosure of which is hereby incorporated herein by reference.

FIELD OF INVENTION

The present invention relates to a surgical system for bone tracking anda method for implant selection utilizing the same, and more particularlyto a surgical system for patella tracking and a method for selecting aproperly-sized patellar implant utilizing the same.

BACKGROUND OF THE INVENTION

Implants for mobile bones require precise implant sizing and selectionto maintain natural bone kinematics. Maintaining natural biomechanics ofjoints is a key requirement for proper rehabilitation of treated bones.For example, a patellar implant must be properly sized to match aresected patella to ensure proper patellar kinematics during flexion andextension of the patella after implantation. Postoperative patellarkinematics must be similar to a patient's preoperative and/orpre-disease state. Improperly sized patellar implants may alter patellarkinematic resulting in patellar maltracking or other complications.

Selecting a patellar implant based on a single or even a plurality ofknee flexion-extension positions may not necessarily ensure properpatellar kinematics over the entire flexion-extension path of a kneejoint. The patellar range of motion through extension and flexioninvolves patellar movement with six degrees of freedom over the courseof flexion-extension cycle. Consequently, matching patellar implantsbased on a single or a plurality of patellar positions may be inadequateto ensure proper patellar kinematics. While anatomically-shaped patellarimplants often provide improved knee rehabilitation, proper implantsizing for anatomically-shaped patellar implants can be especiallychallenging given their complex geometric configuration and range ofmotion.

Patellar overstuffing, i.e., implanting a patellar implant that resultsin a patella which is thicker than the natural patella, can decreasepassive knee flexion and patellar kinematics during knee flexion.Conversely, patellar understuffing, i.e., implanting a patellar implantthat results in a patella which is thinner than the natural patella, maylead to reduced moment arm and subsequently increased quad forcerequirements during patellar flexion-extension leading to increasedcontact pressure on the patella. Consequently, proper patella implantselection to ensure that patella is not overstuffed or understuffed iscritical for proper patellar kinematics.

Therefore, there exists a need for a surgical system for patellatracking and a method for selecting a patellar implant utilizing thesame that overcomes the deficiencies of the prior art.

BRIEF SUMMARY OF THE INVENTION

Disclosed herein are surgical systems for bone tracking and methods forimplant selection.

In a first aspect of the present invention an orthopedic surgery systemis provided. The orthopedic surgery system may include a first tracker,a second tracker and a patellar tracking system. The first tracker maybe configured to contact an unresected patella or a resected patella.The second tracker may be configured to contact a bone. The patellartracking system may be configured to track the first tracker in contactwith the unresected patella in flexion and extension to generate a firstpath representing a patellar range of motion with reference to the bone.The patellar tracking system may be configured to track the firsttracker in contact with the resected patella in flexion and extension togenerate a second path representing a patellar trial range of motionwith reference to the bone. The resected patella may be in contact witha patellar trial.

In accordance with the first aspect, the patellar tracking system mayfurther include a display to depict the patellar range of motion and thepatellar trial range of motion with reference to the bone respectively.The first and second trackers may be detachably secured to the patellaand the bone respectively. The patellar tracking system may include atracking camera to register the positions of the first and secondtrackers.

Further in accordance with the first aspect, the patellar trackingsystem may be configured to register the first tracker secured to theunresected patella in at least a first, a second and a third positionwith reference to the bone to define the first path. The first positionmay be at patellar flexion, the third positon may be at patellarextension and the second position may be located therebetween.

Still further in accordance with the first aspect, the patellar trackingsystem may be configured to register the first tracker secured to theresected patella in contact with the patellar trial in at least a first,a second and a third position with the reference to the bone to definethe patellar trial range of motion. The first position may be atpatellar flexion, the third positon may be at patellar extension and thesecond position may be located therebetween. The bone may be a femur ortibia.

A second aspect of the present invention is a method for selecting apatellar implant. A method in accordance with this aspect of theinvention may include the steps of tracking a location of a pointadjacent an unresected patella to generate a first path, resecting thepatella and placing a patellar trial on a resected surface of thepatella, tracking a location of the point adjacent the resected patellato generate a second path, and displaying and comparing the first pathwith the second path on a display. The first path may be generated bytracking the location of the point adjacent the unresected patella inflexion and extension. The first path may represent a patellar range ofmotion with respect to a bone. The second path may be generated bytracking the location of the point adjacent the resected patella withthe patellar trial in flexion and extension with respect to the bone.

In accordance with the second aspect, the method may further includingthe step of selecting a patellar implant. The step of selecting thepatellar implant may be based on comparing the first path with thesecond path on the display such that the difference between the firstpath and the second path is less than a predetermined value. Thepredetermined value may be 3 mm.

Further in accordance with the second aspect, the step of tracking thelocation of the point adjacent the unresected patella in flexion andextension may include the step of placing a first tracker on the patellaand a second tracker on the bone and using a patellar tracking system togenerate the first path representing the patellar range of motion withreference to the bone. The step of placing the first tracker on theunresected patella may include the step of placing a probe on a checkpost in contact with the unresected patella. The step of placing a firsttracker on the unresected patella may include placing a clamp on theunresected patella. The patellar range of motion may be generated by thestep of registering the position of the first tracker on the unresectedpatella with reference to the bone in at least a first, a second and athird position. The first position may be at patellar flexion, the thirdpositon may be at patellar extension and a second position may belocated therebetween.

Still further in accordance with the second aspect, the step of trackingthe resected patella in flexion and extension may include the step ofplacing the first tracker on the resected patella and the second trackeron the bone and using the patellar tracking system to generate thesecond path representing the patellar trial range of motion withreference to the bone. The patellar trial range of motion may begenerated by the step of registering the position of the first trackeron the resected patella with reference to the bone in at least thefirst, second and third positions.

Still further in accordance with the second aspect, the step ofregistering may include using a tracking camera to register thepositions of the first and second trackers. The step of generating thefirst path may include using the patellar tracking system to generatethe first path between the first, second and third positions of thefirst tracker placed on the unresected patella to define the patellarrange of motion. The step of generating the second path may includeusing the patellar tracking system to generate the second path betweenthe first, second and third positions of the first tracker placed on theresected patella to define the patellar trial range of motion. The stepof displaying and comparing the first path with the second path mayinclude displaying and comparing the first path and the second path on adisplay screen.

In other aspects, the bone may be a femur or a tibia. The predeterminedvalue may correspond to the reduction or elimination of patellaroverstuffing and understuffing

Still further in accordance with the second aspect, the method mayinclude the steps of generating virtual models of the patella and of thebone, virtually tracking a location of a point adjacent a virtuallyresected patella with a virtual patellar trial in flexion and extensionto generate a virtual path representing a virtual trial patellar rangeof motion with respect to the bone, and comparing the first path and thevirtual path to identify the patellar trial.

In a third aspect of the present invention, a check post for anorthopedic probe is provided. A check post in accordance with thisaspect may include an elongated member and an anti-rotation feature. Theelongated member may have a distal end and a proximal end. The distalend may be configured for detachable attachment with a bone or softtissue at a first location. The proximal end may have first and secondrecesses to receive an orthopedic tracker. The anti-rotation feature mayextend from the proximal end of the elongated member. The anti-rotationfeature may include a second elongated member for detachable attachmentwith the bone or tissue at a second location. The check post may notrotate about the first elongated member when the first elongated memberand the anti-rotation feature are attached to the first and secondlocations respectively of the bone or tissue.

In a fourth aspect of the present invention, a patellar trial assemblyis provided. The patellar trial assembly may include an upper bonecontacting surface, a lower surface configured to contact a patella or apatellar implant, and a first and second peg extending from the lowersurface. The first and second peg may be flexible such that the firstand second peg may be pushed towards each other by an external forcesuch that a first distance between distal ends of the first and secondpeg when the external force is applied is less than a second distancebetween the distal ends of the first and second peg when the externalforce is removed.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description, in which reference ismade to the following accompanying drawings:

FIG. 1 is a schematic drawing of a surgical system in accordance with anembodiment of the present invention;

FIG. 2 is a side view schematic depicting trackers of the surgicalsystem of FIG. 1 placed on bone;

FIG. 3 is a graphical representation of a femur and tibia showing afirst path defined by the trackers of FIG. 2 on a display;

FIG. 4 is a graphical representation of a femur and tibia showing asecond path defined by the trackers of FIG. 2 on the display;

FIG. 5 is a graph showing patellar understuffing and overstuffing limitsduring flexion and extension;

FIG. 6 is a flowchart showing the steps for selecting a patellar implantutilizing a patellar trial in accordance with another embodiment of thepresent invention;

FIG. 7 is a flowchart showing the steps for selecting a patellar implantutilizing a virtual patellar trial in accordance with yet anotherembodiment of the present invention;

FIG. 8 is a perspective view of a check post according to anotherembodiment of the present invention,

FIG. 9A is a side view of a patellar trial according to anotherembodiment of the present invention;

FIG. 9B is a top view of the patellar trial of FIG. 9A;

FIG. 10 shows front schematic views of a patellofemoral joint;

FIG. 11A is a front view of a patellar trial assembly according toanother embodiment of the present invention;

FIG. 11B is a front view of a top portion of the patellar trial assemblyof FIG. 11A;

FIG. 11C is a top view of a bottom portion of the patellar trialassembly of FIG. 11A, and

FIG. 12 is a top view of a patella with a hole to receive the patellartrial assembly of FIG. 11A.

DETAILED DESCRIPTION

In describing preferred embodiments of the disclosure, reference will bemade to directional nomenclature used in describing the human body. Itis noted that this nomenclature is used only for convenience and that itis not intended to be limiting with respect to the scope of theinvention.

As used herein, when referring to bones or other parts of the body, theterm “anterior” means toward the front part or the face and the term“posterior” means toward the back of the body. The term “medial” meanstoward the midline of the body and the term “lateral” means away fromthe midline of the body. The term “superior” means closer to the heartand the term “inferior” means more distant from the heart.

Referring to FIG. 1, a first embodiment in accordance with the presentinvention is a surgical system 10 for bone tracking that includes one ormore trackers 100 and a tracking system 200. Tracking system 200includes a position capturing device 210 and a display 220. While thepresent description of surgical system 10 relates to tracking a patella,other surgical systems in accordance with the present invention can beconfigured to track other mobile bones and joints in other embodiments,such as tracking a femur in a hip resurfacing procedure, tracking ahumerus in a shoulder surgery, a talar bone during ankle surgery, etc.

FIG. 2 shows a schematic of trackers 100 located on bone for tracking.

Trackers 100 include a first tracker 102 and a second tracker 104. Firsttracker 102 has a distal tip 106 configured to contact bone, such as apatella 16 as shown, and a plurality of fiducials 110 at a proximal end.Fiducials 110 are spherical-shaped markers configured to be identifiedby position capturing device 210. The number, shape and properties ofmarkers can be varied depending on the type of the position capturingdevice being utilized and the nature of bone being tracked. For example,radiopaque markers can be used with an X-ray or a CT scan positioncapturing device, whereas markers with reflectors can be used with acamera position capturing device. Second tracker 104 has a distal tip108 configured to contact bone, such as a femur 12 as shown, and aplurality of fiducials 112 at a proximal end. Distal tip 108 andfiducials 112 are similar to distal tip 106 and fiducials 110 of firsttracker 102.

As shown in FIG. 2, distal tip 106 of first tracker 102 is placed incontact with patella 16 and distal tip 108 of second tracker 104 isplaced in contact with femur 12. The distal tip of the tracker can beconfigured to be firmly secured and anchored on the bone requiring nomanual support, or it can be configured to allow an operator to manuallyhold the tracker in place during tracking. A clamp or other suitabledevice (not shown) can be used to secure patella 16 prior to placingfirst tracker 102. A navigated patella clamp with a tracker may be usedto intraoperatively inform the surgeon where to place the clamp on thepatella in the manner disclosed in U.S. patent application Ser. No.15/087,202, the disclosure of which is hereby incorporated by referenceherein. Second tracker 104 serves as a reference to track patellarposition during flexion-extension, and consequently can be placed ontibia 14 instead of femur 12 to obtain similar results. While the distaltips of trackers 102, 104 are placed directly on bone in thisembodiment, in other embodiments distal tips can be placed on a checkpost affixed to the bone as more fully described below.

Second tracker 104 is anchored on femur 12 to provide a reference pointto accurately track the location of distal tip 106 of first tracker 102as tibia 14 is moved between flexion and extension indicated bydirection 114. Tracking the location and movement of fiducials 110, 112through flexion and extension allows tracking system 200 to accuratelyrecord and display precise position and orientation of distal tip 106,and therefore, patella 16 through knee flexion-extension cycle. Whilepatellar tracking is disclosed in this embodiment, other embodiments maytrack other mobile bones using surgical system 10 as indicated above.

Referring now to FIGS. 3 and 4, there is shown a display 220 withpatellar tracking information captured by image capture device 210.Display 220 depicts virtual bone models of femur 12′ and tibia 14′. Thevirtual bone models can be preoperatively captured and displayed ondisplay 220, or can be intraoperatively captured during bone tracking.As shown in FIG. 3, distal tip 106 placed on an unresected patella 16 istracked at four different positions 224, 226, 228 and 230 of unresectedpatella 116 as it moves from flexion to extension. A flexion angleindicator 232 allows an operator to identify the precise angle offlexion for each of these positions. Display 220 generates a first path220 defined by positions 224, 226, 228 and 230. First path 220identifies the location of the patella surface at distal tip 106 offirst tracker 102 along flexion and extension of unresected patella 16and depicts the preoperative patellar range of motion during theflexion-extension cycle. Other locations on unresected patella 16, whichare not contacted by the distal tip, can also be derived from distal tip106 location and used to generate first path 220. Other parameters ofthe patellofemoral joint shown in FIG. 2 can also be shown on display220 to record and analyze patellar movement during flexion andextension. For example, varus-valgus angulation 234 with anteroposteriortranslation around an anteroposterior axis and a tibia external angle236 are shown in display 220 of FIG. 3.

With this information, the next step in the procedure is the resectionof the patella. Depending on the geometry of first path 222, surgicalsystem 10 and/or an operator can determine the characteristics of thepatient's patellar range of motion and other relevant metrics tocalculate or optimize the patellar resection depth. Patella resection issubsequently performed utilizing this information.

A second path 238 is defined by positions 224, 226, 228 and 230 withdistal tip 106 on a resected patella with a patellar trial as shown inFIG. 4. Second path 238 is defined by tracking patellar position at aplurality of locations during patellar flexion and extension as firstpath 222 and depicts a patellar trial range of motion. While the patellaitself is not shown, the patella during this step includes an implanthaving a particular thickness anchored to the resected surface. Thisstep therefore trials the location of an exterior portion of themodified patella through flexion-extension. The plurality of locationstracked for second path 238 can be generally the same as the locationsused to generate first path 222, which allows for comparison of the datafrom the unresected and resected procedures. An optimal path 225representing the desired patellar path is also displayed to assist thesurgeon in selecting the appropriate patellar implant. Optimal path 225can be determined by utilizing a contralateral healthy patellar range ofmotion, a patellar range of motion database or other similar methods. Acontralateral healthy patellar range of motion can be obtained byutilizing surgical system 10. As shown here, positions 224, 226, 228 and230 correspond to patellar flexion angles of 13.5°, 19.0°, 47.5° and73.0°, respectively.

First path 222 is overlaid on display 220 for visual comparison betweenthe preoperative patellar range of motion and the patellar trial rangeof motion. A difference 240 between first path 222 and second path 238depicts whether there may be patellar understuffing or overstuffing. Asshown in FIG. 4, second path 238 representing the patellar trial rangeof motion is farther away from first path 222 representing thepreoperative patellar trial range of motion with reference topatellofemoral joint. Hence, the patellar trial range of motion depictedby second path 238 represents an overstuffed patella. Difference 240denotes the level of overstuffing. If second path 238 was closer to thepatellofemoral joint than first path 222, second path 238 wouldrepresent an understuffed patella. The magnitude of difference betweenfirst and second paths denotes the level of overstuffing/understuffingand/or improper alignment.

FIG. 5 shows a graph depicting preoperative patellar range of motion 222and patellar trial ranges of motion 238 and 238′ during patellarflexion-extension cycle. In this example, preoperative patellar range ofmotion is assumed to be identical to optimal path 225—i.e., thepreoperative patellar range of motion represent the optimal patellarrange of motion. Patellar trial range of motion 238 represents anoverstuffed patella whereas patellar trial range of motion 238′represents an understuffed patella. Patellar or patellar trial positionsrecorded at four locations are shown in this graph, but otherembodiments may have fewer or greater number of recorded positionsdepending on the level of tracking accuracy required. Patellaroverstuffing 240 and under patellar understuffing 242 limits aredepicted in FIG. 5. By way of example only and not intended to belimiting in scope, maximum patellar overstuffing and understuffinglimits should be preferably less than 3 mm to maintain the naturalpatellofemoral kinematics. The limits being calculated as the differencebetween two identical tracking positions—i.e., flexion angle, measuredalong a line joining the tracking positions. Correspondingly, patellarimplant selection and patellar resection depths are determined based onthese limits to prevent patellar overstuffing and understuffing. Whilepatellar tracking for patellar overstuffing and understuffing isdescribed in this embodiment, surgical system 10 can be used to trackand analyze other patellofemoral kinematic metrics such as varus-valgusangulation, internal-external rotation of the patella, deviation ofpatellar contact surface from Whiteside's Line 24 as shown in FIG. 10,etc.

Referring now to FIG. 6, there is shown a flowchart depicting a method300 for patellar implant selection according to another embodiment ofthe present invention. In step 300, a preoperative patellar range ofmotion in flexion and extension is tracked and displayed on display 220.As described above, first tracker 102 is placed in contact withunresected patella 16 and second tracker 104 is placed on femur 12 ortibia 14 to provide a reference point. First tracker 102 can also beplaced on a check post that is in contact with bone as more fullydescribed below. A clamp or other suitable device can be placed onpatella 16 to facilitate placement of first tracker 102. Patella 16 isthen moved from flexion to extension to allow image capture device 210to record patella position at a plurality of locations and display thesame as first path 222 on display 220. The number of locations to recordpatellar positions can be varied depending on the accuracy required forthe patellar range of motion.

In step 320, a surgeon performs a patellar resection based on apredetermined resection depth. Resection depth can be preoperativelydetermined by evaluating medical images of the patella orintraoperatively determined by utilizing tacking system 200. A surgeonthen places a suitable patellar trial on the resected patella andperforms step 330. In step 330, first tracker 102 is placed on theresected patella with the patellar trial and patellar positions at aplurality of positions during flexion-extension are recorded anddisplayed on display 200 to defined second path 238. As more fullydescribed above, second path 238 represents the patellar trial range ofmotion for that particularly dimensioned trial implant. In step 340,first path 222 and second path 238 are displayed on display 220 andanalyzed to determined patellar overstuffing or patellar understuffing.If the selected patellar trial results in overstuffing in excess ofpredetermined tolerance limits, the surgeon can select a thinnerpatellar trial and repeat step 330. Similarly, if the selected patellartrial results in understuffing, the surgeon can select a thickerpatellar trial and repeat step 330. Once a suitable patellar trial isidentified and verified to simulate natural or a desired patellarkinematics, a corresponding patellar implant is selected in step 350 andthen implanted. Final testing can be done to ensure proper kinematics ofthe permanent implant.

FIG. 7 shows a method 300′ for patellar selection utilizing a virtualpatellar trial according to another embodiment of the present invention.Method 300′ is similar to method 300 and includes step 310 to track andrecord the preoperative patellar range of motion as described above.However, once the preoperative patellar range of motion is determined,surgical system 10 can virtually determine a patellar resection depth instep 320′. The virtual resection depth can be determined by analyzingimaging of the patellofemoral joint and patient-specific requirements.After a virtual resection depth is determined, a virtual patellar trialtracking 330′ using a virtual patellar trial is performed by surgicalsystem 10. A virtual patellar path analysis with reference to thepreoperative patellar range of motion is performed in step 340′ toidentify and select the required patellar implant by surgical system 10.Therefore, method 300′ provides a surgeon with the resection depth andpatellar implant selection based on patella tracking step 310. Method300′ will aid in various patellar preparation steps such as attachmentoptions for securing a patellar trial to a resected patella, placementof a cutting jig to perform patella resection, etc.

Referring now to FIG. 8, there is shown a check post 400 according toanother embodiment of the present invention. Check post 400 isconfigured to be securely positioned on a mobile body portion such asskin or bone, i.e. the patella, to allow for recording the contactedbody portion in six degrees of freedom via the trackers described above.For example, check post 400 can be placed on a patella to recordextension with mediolateral translation around a mediolateral axis,varus-valgus angulation with anteroposterior translation around ananteroposterior axis, and internal and external rotation withsuperoinferior translation around a superoinferior axis.

Check post 400 includes a first elongated member 402 with a first distaltip 404. Distal tip 404 is shaped to engage with bone or skin andsecurely affix check post 400 to the same. A predrilled bore on bone canaid in securing check post 400 to the bone. The predrilled bore can alsoserve as the anchoring receptacle to receive the patellar implant inorder to minimize patellar resection. Grooves or ridges along elongatedmember 402 are provided to further enhance the connection between checkpost 400 and bone or skin. An anti-rotation feature 414 including asecond elongated member 406 with a second distal tip 408 is provided.Second distal tip 408 is also configured to engage firmly with bone orskin such that check post 400 does not rotate. For example, check post400 will not rotate about a longitudinal axis defined by elongatedmember 402 during patellar extension and flexion when distal tips 404,408 are secured to a patella. A proximal end of check post 400 includesa first recess 410 at a proximal end and a second recess 412 normal tofirst recess 410. Recesses 410 and 412 are shaped to receive and holddistal tips of trackers during patellar flexion and extension. Forexample, distal tips 106 and 108 of the first and second trackers shownin FIG. 2 can be placed in recesses 410 or 412.

Check post 400 can be utilized in method 300 for patellar implantselection described above. Check post 400 can be anchored to unresectedpatella 16 by securing distal tip 404 in the patella. A predrilled boreon unresected patella 16 can also be used to facilitate anchoring checkpost 400 to patella 16. Distal tip 106 can now be placed in recess 410or recess 412. Recess 410 and 412 are configured to receive and hold thedistal tip through the patellar tracking by allowing the distal tip torotate within the recess during flexion-extension cycles withoutbreaking contact between the distal tip and the recess. Check post 400can be similarly used for performing the step of patellar trial tracking330.

FIGS. 9A and 9B show a patellar trial 500 according to anotherembodiment of the present invention. Patellar trial 500 includes apatella contacting surface 502, a femur contacting surface 504 boundedby a medial side 506 and a lateral side 508. Femur contacting surface504 includes a medial sensor 510, a central sensor 512 and a lateralsensor 514. Sensors can be mechanical, electrical, acoustical sensors orany combination thereof configured to detect patellofemoral kinematics.For example, sensors can be configured to track medial-lateraldeflection of a patella 16 shown in FIG. 10. Contact surface area andcontact pressure can be detected by sensors 510, 512 and 514 duringflexion and extension of patellar trial 500 in contact with a resectedpatella (not shown) and/or other anatomical landmarks such as a 18fibula, etc. shown in FIG. 10. This data can be transferred to a displaydevice and compared with patellar tracking data obtained from acontralateral patella or a database containing patellar trackinginformation. Sensors 510, 512, and 514 can be used to determine aquadriceps angle (“Q-angle”) 24 defined as the angle between ananatomical axis 22 and a mechanical axis 20 of patella 16. Q-anglesoutside the prescribed range can indicate a risk of chondramalaciapatella, patella alta or mal tracking of the patella. A surgeon canconfirm proper patellar implant sizing by analyzing the feedback frompatellar trial 500 to ensure that Q-angle 24 is within the desiredrange. While three sensors are shown here, other embodiments can have asingle or a plurality of sensors disposed on and within patellar trial500 to aid in patellofemoral procedures.

Referring now to FIGS. 11A, 11B and 11C, there is shown a patellar trialassembly 600 according to another embodiment of the present invention.Patellar trial assembly 600 includes a top portion 602 and a bottomportion 608. Top portion 608 includes a femur contacting surface 605 anda bottom surface 604. Surface 605 is made of a polymeric material orother suitable material to allow sliding contact with a femur. Bottomsurface 604 can be metallic or other suitably hard material. A peg 606extends from bottom surface 604 and has three prongs. The outer prongshave lateral and medial extensions as best shown in FIGS. 11A and 11B.The prongs of peg 606 are configured to be flexible to allow the outerpongs to be compressed towards the center prong. In other embodiments,the prongs can be circumferentially spaced in a circular orientation.

Bottom portion 608 includes a first surface 614 and a second surface 616as shown in FIG. 11C. A slot 610 is centrally located between firstsurface 614 and second surface 616. Springs 612 disposed on either sideof slot 610 bias first surface 614 away from second surface 616. Peg 606is sized to fit through slot 610 as shown in FIG. 11A. An operator cansqueeze or push first surface 614 and second surface toward each otherto compress peg 606. The compressed peg 606 can then be placed into apredrilled hole 618 on patella 12 shown in FIG. 12. Upon release of thecompressive force on bottom portion 608, first surface 614 and secondsurface 616 are pushed apart by springs 612 and thereby allowing theouter prongs of peg 606 to expand and contact the sides of hole 618.Hole 618 is configured to be slightly smaller than uncompressed peg 616such that patellar trial assembly 600 is now secured to patella 12. Todetach patellar trial assembly 600 from patella 12, an operator cancompress peg 606 by squeezing bottom portion 608 in the same manner asdescribed above and removing patellar trial assembly 600 from thepatella. While a flexible peg attachment mechanism with a compressiveplate is described here, other embodiments can have different attachmentmechanisms such as a peg expander, a peg with a threaded end to engage acorresponding threaded hole in the patella, a vacuum attachmentmechanism, etc. to facilitate convenient attachment and detachment ofpatellar trials.

Furthermore, although the invention disclosed herein has been describedwith reference to particular features, it is to be understood that thesefeatures are merely illustrative of the principles and applications ofthe present invention. It is therefore to be understood that numerousmodifications, including changes in the sizes of the various featuresdescribed herein, may be made to the illustrative embodiments and thatother arrangements may be devised without departing from the spirit andscope of the present invention. In this regard, the present inventionencompasses numerous additional features in addition to those specificfeatures set forth in the paragraphs below. Moreover, the foregoingdisclosure should be taken by way of illustration rather than by way oflimitation as the present invention is defined in the examples of thenumbered paragraphs, which describe features in accordance with variousembodiments of the invention, set forth in the paragraphs below.

1. An orthopedic surgery system comprising: a first tracker configuredto contact an unresected patella or a resected patella; a second trackerconfigured to contact a bone; and a patellar tracking system configuredto track the first tracker in contact with the unresected patellathrough a flexion and extension motion to generate a first pathrepresenting a patellar range of motion with reference to the bone, andto track the first tracker in contact with the resected patella throughflexion and extension motion to generate a second path representing apatellar trial range of motion with reference to the bone, the resectedpatella being in contact with a patellar trial.
 2. The orthopedicsurgery system of claim 1, wherein the patellar tracking system furtherincludes a display to depict the patellar range of motion and thepatellar trial range of motion with reference to the bone.
 3. Theorthopedic surgery system of claim 2, wherein the first and secondtrackers are detachably secured to the patella and the bonerespectively.
 4. The orthopedic surgery system of claim 3, wherein thepatellar tracking system includes a tracking camera to register thepositions of the first and second trackers.
 5. The orthopedic surgerysystem of claim 1, wherein the patellar tracking system is configured toregister the first tracker secured to the unresected patella in at leasta first, a second and a third position with reference to the bone todefine the first path, the first position being at patellar flexion, thethird positon being at patellar extension and the second position beinglocated therebetween.
 6. The orthopedic surgery system of claim 1,wherein the patellar tracking system is configured to register the firsttracker secured to the resected patella in contact with the patellartrial in at least a first, a second and a third position with thereference to the bone to define the patellar trial range of motion, thefirst position being at patellar flexion, the third positon being atpatellar extension and the second position being located therebetween.7. A method for selecting a patellar implant comprising: tracking alocation of a point adjacent an unresected patella in flexion andextension motion of the knee to generate a first path representing apatellar range of motion with respect to a bone; resecting the patellaand placing a patellar trial on a resected surface of the patella;tracking a location of the point adjacent the resected patella with thepatellar trial in flexion and extension to generate a second pathrepresenting a patellar trial range of motion with respect to the bone;and displaying and comparing the first path with the second path on adisplay.
 8. The method of claim 7, further including the step ofselecting a patellar implant.
 9. The method of claim 8, wherein the stepof selecting the patellar implant is based on comparing the first pathwith the second path on the display such that the difference between thefirst path and the second path is less than a predetermined value. 10.The method of claim 7, wherein the step of tracking the location of thepoint adjacent the unresected patella in flexion and extension includesthe step of placing a first tracker on the patella and a second trackeron the bone and using a patellar tracking system to generate the firstpath representing the patellar range of motion with reference to thebone.
 11. The method of claim 10, wherein the step of placing a firsttracker on the unresected patella includes the step of placing a probeon a check post in contact with the unresected patella.
 12. The methodof claim 11, wherein the patellar range of motion is generated by thestep of registering the position of the first tracker on the unresectedpatella with reference to the bone in at least a first, a second and athird position, the first position being at patellar flexion, the thirdpositon being at patellar extension and a second position being locatedtherebetween.
 13. The method of claim 12, wherein the step of trackingthe resected patella in flexion and extension includes the step ofplacing the first tracker on the resected patella and the second trackeron the bone and using the patellar tracking system to generate thesecond path representing the patellar trial range of motion withreference to the bone.
 14. The method of claim 13, wherein the patellartrial range of motion is generated by the step of registering theposition of the first tracker on the resected patella with reference tothe bone in at least the first, second and third positions.
 15. Themethod of claim 14, wherein the step of registering includes using atracking camera to register the positions of the first and secondtrackers.
 16. The method of claim 15, wherein the step of generating thefirst path includes using the patellar tracking system to generate thefirst path between the first, second and third positions of the firsttracker placed on the unresected patella to define the patellar range ofmotion.
 17. The method of claim 16, wherein the step of generating thesecond path includes using the patellar tracking system to generate thesecond path between the first, second and third positions of the firsttracker placed on the resected patella to define the patellar trialrange of motion.
 18. The method of claim 17, wherein the step ofdisplaying and comparing the first path with the second path includesdisplaying and comparing the first path and the second path on a displayscreen.
 19. The method of claim 7, further comprising: generatingvirtual models of the patella and of the bone; virtually tracking alocation of a point adjacent a virtually resected patella with a virtualpatellar trial in flexion and extension to generate a virtual pathrepresenting a virtual trial patellar range of motion with respect tothe bone; and comparing the first path and the virtual path to identifythe patellar trial.
 20. A patellar trial assembly comprising: an upperbone contacting surface; a lower surface configured to contact a patellaor a patellar implant, and a first and second peg extending from thelower surface, the first and second peg being flexible such that thefirst and second peg can be pushed towards each other by an externalforce such that a first distance between distal ends of the first andsecond peg when the external force is applied is less than a seconddistance between the distal ends of the first and second peg when theexternal force is removed.